Gamma-ray Burststo take center stage at international meeting

Gamma-ray bursts to take center stage at international
meeting

5th Huntsville Gamma Ray Burst Symposium
set for Oct. 18-22

Oct.
11, 1999: Gamma-ray bursts used to be a rather esoteric field
of astrophysics. Bursts were interesting, partly because they
were so elusive. But they were a quirk that didn't command the
same attention as bigger, better-known phenomena. Today they
often command significant observing time by the Hubble Space
Telescope and the Keck Observatory, the two optical telescopes
most in demand by astronomers.

Observations and discoveries over the last three
years have made gamma-ray bursts front page news, reshaping our
perception of how they fit into the grand scheme of the universe.

Later this month, more than 200 scientists will gather to
discuss their findings and their plans for unraveling more about
these mysterious bursts of energy.

"The exciting thing is that we will have scientists from
lots of different astronomical fields, not just high-energy astrophysics,"
said Dr. Valerie Connaughton, a member of the organizing committee
for the 5th Huntsville Gamma Ray Burst Symposium. "We also
have optical [visible light] and radio astronomers planning to
attend."

Connaughton is an
astrophysicist working at NASA's Marshall Space Flight Center.
The 5th Huntsville Gamma Ray Burst Symposium will be held Oct.
18-22 at the Huntsville Hilton Hotel under the sponsorship of
NASA/Marshall and the Universities Space Research Association.

Left: Artwork for 5th Huntsville Gamma Ray Burst Symposium
symbolizes the brightness profiles of bursts and their distribution
across the sky. Credit: NASA/Marshall.

Gamma-ray bursts have puzzled scientists since they were discovered
in the late 1960s by satellites watching for nuclear weapons
tests in space. They recorded bursts of radiation in gamma rays,
the highest part of the electromagnetic spectrum, but not lower
down. And the bursts appeared to be coming from outside the solar
system. Their appearances and locations were random and not associated
with any known object.

As other spacecraft observed bursts and the puzzle deepened,
NASA decided to include a dedicated instrument, the Burst and
Transient Source Experiment, on the Compton Gamma Ray Observatory.

"BATSE would detect bursts with unprecedented sensitivity
so that by studying their distribution in the sky, we could establish
their origins in our galaxy," Connaughton said of early
hopes for BATSE.

Instead, scientists got an even deeper mystery. Bursts flashed
and faded in a matter of seconds or minutes, too quick to aim
a telescope for follow-up observations. And they appeared to
be distributed outside the galaxy and probably deep in the universe.

The big break came in 1997 when Dr. Jan van Paradijs of the
University of Amsterdam, using observations by the Beppo SAX
satellite and ground-based observatories, tied a burst on Feb.
28, 1997 to a source deep in space. The Feb. 28 burst was the
big news at the 4th Huntsville Gamma Ray Burst Symposium in September
1997.

Right:
Yellow circles highlight positions of four BATSE instruments
on the Compton Gamma Ray Observatory. The other four are on the
lower side of the spacecraft. This gives BATSE a continual view
of the entire sky, as symbolized by the octahedron (left). Credit:
NASA/Marshall.

"The sense of the community is that the doubt is over,"
said Dr. Chip Meegan, a BATSE coinvestigator at NASA/Marshall,
before the 1997 symposium. "Gamma ray bursts are cosmological."
That means that instead of coming from within our galaxy or even
immediately around the galaxy, they are deep in space, probably
more than 8 billion light years away (by comparison, our galaxy
is about 150,000 light years across).

"But they're still very strange," Meegan continued.
"All of the questions about them being cosmological are
still there." The principal question is, what produces so
much energy?

The questions still stand and will be discussed, along with
findings from the past two years of observations at the 5th Huntsville
Gamma Ray Burst Symposium.

Left:
A galactic map shows the locations of 2,512 bursts detected by
BATSE since April 1991. A simple mathematical analysis demonstrates
that the distribution is random, thus showing that bursts are
scattered throughout the universe and not associated with our
galaxy (that would be indicated by bursts clustering along the
galactic equator). The array of clumps and open spots is a result
of this randomness. A truly even distribution would be suspect.
Larger versions are available from the BATSE web site. Credit:
NASA/Marshall.

"The supernova crowd will be there since we have
a tentative association between supernovae and gamma-ray bursts,"
Connaughton said. "There's been lots of speculation by astronomers
active in supernova research as well as those active in gamma-ray
bursts."

If the two are associated, then what special conditions lead
to a supernova expending such a phenomenal amount of energy mainly
in the gamma-ray spectrum yet hiding or muting itself in visible
light until weeks later?

In January, the Robotic Optical Transient Search Experiment
at Los Alamos National Laboratory, cued by BATSE, caught the
optical flash within 20 seconds of a gamma-ray flash being recorded.
But many other attempts to catch optical transients coincident
with the gamma-ray emissions have been fruitless.

"There is a really intense debate as to whether these
optical flashes happen with all bursts," Connaughton noted.

Another topic that has grown enough to warrant its own discussion
is Soft Gamma Repeaters (SGRs). Where true gamma-ray bursters
are distant and never repeat (the blast is so energetic that
it shreds the source object), SGRs are within our galaxy and
repeat at unpredictable times.